In a third generation mobile communication system such as IMT-2000, signals such as pilot signals and data signals are transmitted according to code division multiple access (CDMA). In such a communication system, pilot signals are used for channel estimation and play a central role in reception quality assurance. Therefore, pilot signals are often transmitted with a power level higher than that of other signals (power boost).
Meanwhile, in a future mobile communication system succeeding the third generation system, it is planned to use a wider frequency band and to use OFDM for downlink to improve the quality, speed, and capacity of the system (see, for example, 3GPP, TR25.814). Also in such a future mobile communication system (for example, Long Term Evolution (LTE)), it is preferable to improve the accuracy of channel estimation based on pilot signals and therefore it is preferable to transmit pilot signals with a higher power level than that of other signals (such as data signals). Pilot signals may also be called, for example, reference signals, training signals, and known signals.
In OFDM, the total power level necessary to transmit signals in a time slot is obtained by adding up power densities of data mapped to all subcarriers. If a total power level Pall exceeds a maximum allowable transmission power level PMAX of a transmitter, undesired distortion is caused in the transmission signal. Therefore, the total power level Pall is preferably limited to or below the maximum allowable transmission power level PMAX. However, while it is unknown what types of data signals are to be mapped to which subcarriers before the data signals are finally determined, it is difficult to limit the total power level Pall to or below the maximum allowable transmission power level PMAX after the data signals are mapped to the subcarriers.
Also, since it is not necessary to always send reference signals, each time slot may or may not include reference signals. Therefore, when the reference signals are to be sent with a high power level, the total power levels Pall necessary for respective time slots differ greatly. Here, it may be possible to reserve a fairly large margin for the power density of data to be mapped to each subcarrier such that the total power level Pall of each time slot does not exceed the maximum allowable transmission power level PMAX. However, this approach may reduce the amplification efficiency of a transmission power amplifier.